As of now, it is not possible to create custom Niagara Systems using the "Unique Particle ID" emitter as a base. There seems to be an issue with the thumbnail. Other bases might have similar issues, but I didn't test it. When assigning the NS as a "Custom Visual Effect" in the VFX Spawner Device, I'm getting the following error when validating assets:
"Warnings logged while validating asset Texture2D /Engine/Transient.NiagaraSystem_2:ParticlesUniqueID.ThumbnailTexture contains no miplevels! This could happen if this texture is a thumbnail and hasn't been generated (Format: 0)"
Since we don't get access to the Engine folder UEFN, I don't think there is anything that we can do about this - or is there? If there is a way, I'd be happy to know, since I was hoping to be using these VFX in my project.
The team is aware and working on a fix. Thanks again for the report!